Quantitative determinations of equilibrium and kinetic acidities of hydrocarbons and their simple derivatives are used to evaluate and develop physical organic concepts and theories of effects of structure on reactivity. Kinetic acidities are determined by hydrogen isotope exchange reactions with methanolic sodium methoxide and with lithium or cesium cyclohexylamide in cyclohexylamine. Equilibrium acidities are determined by comparisons with indicator hydrocarbons using lithium or cesium cyclohexylamide in cyclohexylamine. Classes of compounds being studied by these methods include substituted fluorenes, fluorinated bicyclic hydrocarbons and other aliphatic hydrocarbons, substituted benzenes, cycloalkenes, di- and tri- arylmethane derivatives and organic sulfur compounds.